1. Field of the Invention
The present invention relates to a charged particle beam apparatus, and more particularly to a charged particle beam apparatus capable of improving magnification accuracy.
2. Description of the Related Art
For a charged particle beam apparatus represented by a scanning electron microscope, high focusing accuracy and high magnification accuracy are required. For example, an electron beam needs to be accurately focused with a desired gap between the surface of a sample and a focal point at which the beam is focused. In addition, an enlarged image of the sample needs to be accurately displayed at a desired magnification. Therefore, it is necessary for a lens (provided in the charged particle beam apparatus) for focusing the electron beam to be controlled with high accuracy to ensure that the focal point of the electron beam is located at a desired position.
For example, the magnification of the scanning electron microscope is defined by the position of the sample, i.e., a working distance (WD). The magnification of the scanning electron microscope is represented as a ratio of an image displacement that is displayed by a CRT or on a picture and corresponds to a movement of a probe scanning the surface of the sample to the distance of the movement of the probe scanning the surface of the sample. In order to accurately display the magnification of the scanning electron microscope (SEM), it is necessary that the WD be set to a defined distance. The WD is determined based on the distance between an objective lens and the position of an object point of the objective lens and on an excitation current applied to the objective lens. It is therefore necessary to accurately control the position of the object point of the objective lens in order to set the WD to the defined distance.
The charged particle beam apparatus is used for a wide variety of purposes such as observation of a sample with high resolution, measurement of the size of the sample, inspection of appearance of the sample, and analysis of the sample. There is an optical system optimized for each of the purposes. Therefore, when the charged particle beam apparatus is used for the purposes, it is necessary that the focal point of the electron beam focused by the lens largely vary.
The lens for focusing the electron beam, which is provided in the charged particle beam apparatus, needs to be set under a wide variety of conditions with high accuracy.
With the increased needs for measurement using a large probe current, in many cases a sample is irradiated with a large current to measure the sample. With such a situation, it has been noticeable that a magnification displayed by a charged particle beam apparatus is disadvantageously different from the actual magnification.
In a conventional technique for correcting an error in the magnification, for instance, a displacement of a focal point and uncertainty in a setting value for each lens are accurately measured for each of setting conditions for purposes for use of the charged particle beam apparatus. The measured results are stored in the charged particle beam apparatus. One or more of the measured results suitable for a certain condition for use of the device are read out to correct the lens.
In a technique disclosed in JP-A-2002-15691, a certain magnification is specified, and a scanned image is displayed. In addition, an actual magnification is obtained by using a reference sample, and the displayed specified magnification is changed to the actual magnification.